The present invention relates to an electrically weldable pipe joint for joining thermoplastic pipes such as polyethylene pipes and a production method thereof, and more specifically relates to an inner-less electrically weldable pipe joint injection-molded using a core having a resistance heating wire directly wound on the outer surface which defines the inner surface of the pipe joint.
Plastic pipes to be connected are inserted into the hollow interior of an electrically weldable pipe joint, and then electric power is supplied through the resistance heating wire embedded in the inner surface of the pipe joint. The plastic material of the outer surface of the pipes being connected and the inner surface of the pipe joint is partly melted by the generated resistance heat to tightly connect the pipe joint to the pipes.
Japanese Patent Publication No. 62-12437 discloses a production method of a pipe joint with an inner body on the inner peripheral surface thereof. In this method, a thin cylindrical plastic inner body with a grooved surface, which constitutes the inner surface of the pipe joint body, surrounds the outer surface of the core. After winding a resistance heating wire on the inner body along the bottoms of the groove, the core with a surrounding inner body is placed in a mold. Then, a molten resin is injected to a cavity between the mold inner surface and the outer surface of the inner body to injection-mold the electrically weldable pipe joint.
Since the resistance heating wire is received in the groove on the inner body outer surface, it has been expected that the resistance heating wire does not change its position during the injection molding to maintain the distance between each turn of the resistance heating wire constant.
However, this method requires an additional process for producing the inner body prior to injection-molding the pipe joint. In addition, since the inner body has a thin wall, the wall and the webs for defining the grooves are melted due to an injection pressure and a heat from the injected molten resin, and as a result, the wire frequently changes its position during the injection molding. In some cases, a turn of wire contacts with or overlaps another turn to cause an interturn short-circuit of the resistance heating wire to prevent sound welds of the pipes.
Japanese Patent Laid-Open No. 2-30517 discloses a production method of an inner-less electrically weldable pipe joint as shown in FIG. 16. In this method, a two-part solid core 5, 5 with a resin-coated resistance heating wire 3, each of both ends thereof is connected to each terminal pin 4, 4, wound directly on the outer surface thereof, which defines the inner surface of the pipe joint body, is placed in a mold 7. The pipe joint body is injection-molded by injecting a molten resin into a cavity between the mold 7 and the core 5. After the molding is completed, the molded pipe joint is removed from the mold 7, and the cores 5, 5 are axially withdrawn form the molded pipe joint in the opposite directions to obtain an electrically weldable pipe joint having the resin-coated wire 3 embedded in the inner surface thereof.
In the known method of producing the inner-less electrically weldable pipe joint mentioned above, the resin-coated wire 3 wound on the outer surface of the core 5 changes its position due to the injection pressure and the heat from the injected molten resin to produce an electrically weldable pipe joint having a wire embedded in the inner surface thereof with uneven spacing between each tun of the wire 3 and/or a wire out of the intended positions. Therefore, during the welding operation, an interturn short-circuit occurs between the closely positioned turns of the wire 3 or between the contacting turns of wire 3 to prevent sound weld of the pipes.
Therefore, the resistance heating wire 3 used in the above method should be coated with a thermoplastic insulating resin to avoid the short-circuit. However, the short-circuit cannot be avoided completely even when such a resin-coated wire is used because the resin coating is melted due to the injection pressure and the heat from the injected molted resin. In addition, since the resin coating is difficult to have a constant thickness over the full length of the wire, the distance between the resistance heating wire and the inner surface of the pipe joint is variable to result in an uneven heating.
In the known electrically weldable pipe joint with or without the inner body, the resistance heating wire is completely embedded in the inner surface thereof. Therefore, during the welding operation, the resin of the pipe joint body in the immediate vicinity of the wire is first heated, and then the heat from the resistance heating wire is conducted to the pipes being connected to melt the resin of the outer surface of the pipes. This delayed heating requires an excess of electric power and an prolonged period of time for completing the welds. Therefore, the pipe joint body is heated excessively before the outer surface of the pipes being connected is melted to deteriorate the resin in the vicinity of the resistance heating wire or deform the pipe joint body.